The invention is directed to the field of liquid-type photoresists and the method of electrophoresis for depositing such films on conductive surfaces. Electrophoresis refers to the motion of charged particles or molecules through a liquid medium under the influence of an applied electric field. Electrophoretic deposition or electrodeposition is conducted in an electrolytic cell where the surfaces of the conductive material, to be coated with the migrating charged particles, serves as one electrode. The electrodeposition of cationic polymers (positively charged polymers) on the surface of a negatively charged electrode (cathode) is referred to as cataphoresis, while the electrophoretic deposition of anionic polymers (negatively charged polymers) on the surface of a positively charged electrode (anode) is referred to anaphoresis.
Coating metal objects by electrophoresis is well known and widely used to paint metal surfaces such as automobiles. Electrophoresis has been used to prepare electrical components, such as resistors and capacitors, that are integral to printed wiring boards. It is also known to use electrophoresis to deposit non-photoactive thermally curable coatings. Electrodeposition of photosensitive coatings by anaphoresis is disclosed in U.S. Pat. No. 3,738,835.
U.S. Pat. No. 4,845,012 discloses photocurable electrodeposition coating compositions for printed circuit photoresist film comprising a water-soluble or water-dispersible polymerizable unsaturated polymer having an acid value of 20 to 300, an unsaturation equivalent of 150 to 3000 and a number average molecular weight of not less than 300, a water-insoluble polymerizable initiator, and a chelating agent. .beta.-diketone is listed as an example of a chelating agent.
Existing anodic electrodeposited resists suffer from poor alkaline etch survivability. The exposed resist patterns tend to be attacked by pH 8-9 ammoniated cupric chloride etch solutions commonly used in the printed wiring board (PWB) industry to etch wiring patterns in a copper clad PWB substrate. A need exists for an additive which when added to electrodeposition resist emulsion formulations improves alkaline etch resistance, without destabilizing the emulsion or having a deleterious effect on the coating quality. Tack characteristics of the electrodeposited resist coating also need improvement.
U.S. Pat. No. 4,943,513 discloses the addition of multivalent metal chelate salts of weakly acidic organic compounds along with other anionic organic or inorganic groups to dry film photoresist formulations to reduce cold flow.
Ionizable materials such as metal salts are known to adversely affect electrodeposited coating quality. Evidence of this is found on page 8 of Electrodeposition And Radiation Curing Of Coatings, by M. W. Ranney, Noyes Data Corporation, 1970 and U.S. Pat. No. 3,444,064.
Surprisingly and unexpectedly, addition of specific metal salts of .beta.-diketones, of the structure shown below, to electrodeposition resist emulsion formulations can lead to a coating with improved alkaline etch resistance, without destabilizing the emulsion or having a deleterious effect on the coating quality. Also, such electrodeposited resist coating can result in reduced tack.